The present invention generally relates to optical recording and reading, and more particularly, to a near-field optical storage system.
Optical storage can be used to achieve high areal density data storage by using a tightly focused laser beam. For example, electro-optical data storage systems based on magneto-optical materials can be configured to produce an areal data density on the order of one gigabit per square inch.
A monochromatic optical beam can be focused to a small spot by using an optical focusing module with a large numerical aperture. This can produce a minimum spot size on the order of one wavelength due to the diffraction limit. The areal density of an optical storage device, in principle, is limited by this diffraction-limited spot size.
One technique for increasing the areal data density is to reduce the spot size of a beam within the diffraction limit by using light sources of short wavelengths, such as lasers toward the blue end of the optical spectrum.
Another technique focuses an optical beam onto the flat surface of a solid transparent material with a high refractive index. The diffraction-limited focused spot size is hence reduced by a factor of the refractive index compared to the spot size in air.
The optical energy can be coupled between the optical focusing module and the surface of an optical recording medium via evanescent fields by placing the medium surface near the surface of the solid material, typically closer than one wavelength, to form a near-field configuration. For example, U.S. Pat. No. 5,125,750 to Corle and Kino discloses a near-field optical recording system based on a solid immersion lens. In a near-field configuration, the numerical aperture of the optical focusing module can be greater than unity which is beyond the diffraction limit in air.
The present invention is embedded in an electro-optical data storage system in a near-field configuration. This system includes an optical train which has a near-field lens for coupling optical energy to and from a recording layer in an optical storage medium. The near-field lens is spaced from the surface of the medium by an air gap typically less than one wavelength in thickness. The optical coupling between the near-field lens and the optical medium is effected by both the optical propagation and evanescent coupling through the air gap.
The optical train of the system is preferably configured to focus an optical beam at a location beyond the position of the recording layer in the optical medium by a desired defocus distance in the absence of the optical medium in order to achieve a minimum or significantly reduced beam spot size on the recording layer in presence of the optical medium. This increases the storage density on the recording layer. The defocus distance is determined by properties and configurations of the objective lens, the near-field lens, the air gap, and the optical medium.
One embodiment of the system includes an objective lens to receive and focus a collimated beam to the near-field lens. The objective lens and the near-field lens are spaced from each other to achieve the desired defocus at or near the exiting surface of the near-field lens so that the beam spot at the recording layer is minimized or significantly reduced.
Another embodiment uses an aspherical objective lens to include effects of refraction of the beam at the interface of the near-field lens and the air gap and spherical aberrations of the objective lens and the near-field lens to achieve the desired defocus.
Yet another embodiment uses a divergent beam instead of a collimated beam to impinge on the objective lens. The amount of divergence is set at a predetermined value to achieve the desired defocus.
Alternatively, the near-field lens may be replaced by a substantially transparent high-index material to provide the near-field coupling to the optical medium.
These and other aspects and advantages of the present invention will become more apparent in light of the following detailed description, the accompanying drawings, and the appended claims.